1. Field of the Invention
The invention relates to a fluidised bed reactor for the gasification and/or pyrolysis of solid fuels, preferably biomass, having a heater for heating the fluidised bed of the fluidised bed reactor, the heater having at least one cavity. The invention furthermore relates to a method for the gasification and/or pyrolysis of solid fuels, preferably biomass, in a fluidised bed reactor, preferably of the type mentioned above.
2. Description of Related Art
The first step in the production of synthesis gas from biomass by means of steam reforming is endothermic. Energy must therefore be input into the process. This can be done by partial combustion of the biomass. The delivery of oxygen into a reactor at a temperature level of about 800° C. is not simple, however, because owing to a large oxygen supply, temperatures can be reached locally which are so high that they cause melting of the ash components of the biomass. For this reason, either the oxygen must be diluted with steam or nitrogen or the biomass must be provided as a small fraction in the inert bed material in the form of small coke particles, which transmit the heat to the bed material. Fluidised bed reactors are therefore usually operated with air.
The best-known industrial plant of this type is located in Güssing, Austria (see: Zweibett Wirbelschichtvergasung in Güssing mit 2 MWel/4.5 MWth [Two-bed fluidised bed gasification in Güssing with 2 MWe/4.5 MWth]; R. Rauch, H. Hofbauer; Holzenergiesymposium [Wood energy symposium] 18 Oct. 2002, ETH Zurich, Switzerland). This plant has two fluidised bed reactors, which are connected to one another by a sand circuit. The reactor, referred to as a burner, is operated with air. Here, the coke particles are burnt and the circulating sand bed is heated to approximately 950° C. The temperature of the coke particles under these oxidising conditions may be more than 1100° C. This type of reactor can therefore be operated safely only with wood. Using biomass of the crop type, owing to its low ash melting point, would lead to clumping of the sand.
For pressurised gasification, as would be advantageous for the production of hydrogen, this type of reactor is less suitable.
In principle, fluidised bed reactors may also be operated autothermally by direct delivery of pure oxygen. In practice, however, the ash melting point of biomass is exceeded in this case even if the oxygen is diluted to one half with steam. Autothermal operation then requires a particular configuration of the oxygen feed, as disclosed in DE 102 42 594 A1, and special extraction for the molten ash. This ash cannot be reused as inorganic fertiliser.
With pure oxygen, neither allothermal operation as of the Güssing type nor autothermal operation by injecting pure oxygen is possible without exceeding the ash melting point. Even an oxygen content of 21% (air) is problematic.